Silver halide grains having two or more parallel twinning planes have a tabular form (hereinafter referred to as tabular grains). The tabular grains have the following photographic characteristics.
(1) The ratio of the surface area of the grain to the volume thereof (hereinafter referred to as specific surface area) is high, and hence large amounts of sensitizing dyes can be adsorbed by the surface of the grain. Accordingly, color-sensitized sensitivity is relatively high. PA1 (2) When emulsions comprising the tabular grains are coated on a support and dried, the grains are arranged parallel to the surface of the support. Accordingly, light scattering caused by the grains can be reduced, and sharpness and resolving power can be improved. Further, the thickness of the coated layer can be reduced by the above arrangement, and sharpness can be improved. PA1 (3) The rate of development is rapid because the specific surface area is large. PA1 (4) Covering power is high, and hence the amount of silver used can be saved. PA1 1) the tabular grains have a wide grain size distribution in terms of the diameter of the circle of the corresponding projected area, and PA1 2) cylindric grains, tetrapod-form grains, singlet twin grains and grains having non-parallel twinning planes are mixed with the tabular grains. PA1 1) high contrast (namely, high gamma value) on the characteristic curve can not be expected; PA1 2) when emulsions comprising larger size grains and smaller size grains are chemical-sensitized, it is difficult that both the larger size grains and the smaller size grains are simultaneously chemical-sensitized best because the larger size grains and the smaller size grains are different in the optimum conditions from each other; and PA1 3) an interlayer effect can not be sufficiently utilized. A multi-layer coat comprising an upper layer containing the monodisperse larger size grains and a lower layer containing the monodisperse smaller size grains provides high sensitivity from the viewpoint of the utilization of light in comparison with the emulsion coated layer wherein the larger size grains and the smaller size grains are mixed with each other. PA1 [1] A silver halide emulsion prepared in the presence of at least one member of polymers comprising at least one repeating unit derived from at least one member of monomers represented by the following formula (1) ##STR2## wherein R.sup.1 represents a hydrogen atom or a lower alkyl group; R.sup.2 represents a monovalent substituent group; R.sup.3 represents an alkylene group having 3 to 10 carbon atoms; L represents a bivalent bonding group; and n represents the mean number of a repeating unit represented by --R.sup.3 --O-- and is a number of at least 4, but not more than 200. PA1 [2] A silver halide emulsion as described in [1] above, wherein the polymers are copolymers comprising at least one repeating unit derived from at least one member of the monomers represented by the formula (1) and at least one repeating unit derived from at least one member of monomers represented by the following formula (2) ##STR3## wherein R.sup.4 represents a hydrogen atom or a lower alkyl group; R.sup.5 represents a monovalent substituent group; L' represents a bivalent bonding group; and m represents the mean number of a repeating unit represented by --CH.sub.2 CH.sub.2 O-- and is a number of at least 4, but not more than 200. PA1 [3] A silver halide photographic emulsion as described in [1] above, wherein the silver halide emulsion comprises tabular grains having an aspect ratio of at least 2 and a coefficient of variation in a grain size distribution in terms of the diameter of the corresponding circle of 15% or lower. PA1 [4] A silver halide photographic material comprising a support having thereon at least one light-sensitive silver halide emulsion layer, wherein the silver halide emulsion layer contains a silver halide emulsion as described in [1] above.
The tabular grains have many advantages as described above, and hence the tabular grains have been conventionally used in marketing photographic materials having high sensitivity.
JP-A-58-113926 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-58-113927 and JP-A-58-113928 disclose emulsions comprising grains having an aspect ratio of 8 or higher. The term "aspect ratio" as used herein refers to the ratio of the diameter of the tabular grain to the thickness thereof. The diameter of the grain is defined at the diameter of a circle having an area equal to the projected area of the grain. The thickness of the grain refers to a distance between two parallel principal surfaces constituting the tabular grain.
Tabular grains conventionally prepared have poor monodispersibility as shown in Examples of the above-described patent specifications. This means that
Accordingly, the tabular grains have disadvantages in that
Accordingly, many attempts have been made to obtain monodisperse tabular grains as disclosed in many patent specifications. For example, JP-A-52-153428 discloses the preparation of monodisperse tabular grains. In the method described in this patent specification, however, AgI crystal must be used as the nucleus, and the proportion of the tabular grains contained in the resulting grains is small. JP-A-55-142329 discloses the growth conditions of grains for obtaining monodisperse tabular grains. However, the proportion of the tabular grains contained in the resulting grains is small. JP-A-51-39027 discloses a method wherein monodisperse twin grains are ripened by adding solvents for silver halide after nucleation, and the grains are then grown. In this method, the proportion of the tabular grains contained in the resulting grains is small, and the resulting tabular grains have a low aspect ratio. JP-A-61-112142 also discloses the preparation of monodisperse twin grains as disclosed in the above patent specification, as a grain forming process. In this patent, however, since spherical grains are used as a seed crystal, only tabular grains having an aspect ratio of 2.2 or below are formed, and only tabular grain emulsions wherein the proportion of the tabular grains is low are obtained. French Patent 2,534,036 discloses a method wherein monodisperse tabular grains are ripened without using any solvent for silver halide after nucleation. The resulting tabular grains have a coefficient of variation (a value obtained by dividing the standard deviation (in terms of the diameter of the corresponding circle) by the mean grain size (in terms of the average diameter of the corresponding circles) and multiplying the quotient by 100) of 15%. When calculated from the photographs of the grains described in Examples of this patent specification, triangular tabular grains account for at least 50% of the entire projected areas of the entire grains. The triangular tabular grains refer to grains which have three parallel twinning planes on the principal surface according to J. E. Maskasky, J. Imaging Sci., 31 (1987), pp. 15-26.
JP-A-63-11928, JP-A-63-151618 and JP-A-2-838 disclose monodisperse tabular grains including hexagonal tabular grains. The hexagonal tabular grains are tabular grains having two parallel twinning planes, unlike the above-described triangular tabular grains. There is disclosed in Example 1 of the aforesaid JP-A-2-838 that monodisperse tabular grains comprise grains having such a proportion that tabular grains having two parallel twinning planes account for 99.7% of the entire projected areas of the entire grains and have a coefficient of variation in terms of the diameter of the corresponding circle of 10.1%.
U.S. Pat. Nos. 5,147,771, 5,171,659, 5,147,772 and 5,147,773 disclose a process for preparing monodisperse tabular grains by allowing polyalkylene oxide block copolymers to be present during nucleation. EP-A-514742 discloses monodisperse tabular grain emulsions comprising grains having a coefficient of variation of 10% or lower. In all of Examples of this patent specification, the above-described polyalkylene oxide block copolymers are used.
However, when tabular grains are prepared according to the method described in Examples of the above-described patent specification, there are formed tabular grains having a distorted form wherein the six sides of a sexangle are randomly different in the length from one another, though monodisperse tabular grains are obtained.
It is generally known that an ideal picture element arrangement is in the form of a honeycomb structure when the arrangement of each picture element is examined by an image sensor. Tabular grains having a regular hexagonal form are preferred (see, L. C. Dainity and R. Shaw, Image Science, Academic Press, London, 1974). Accordingly, it has been demanded to provide hexagonal tabular grains having a uniform hexagonal form. It has been confirmed that the monodisperse tabular grain emulsions prepared according to the above-described patent specifications have photographic advantages in that properties with regard to high contrast and graininess are improved. However, an improvement in the sensitivity/fog ratio is still insufficient. Accordingly, a further improvement has been demanded.